Chapter 13 – Ions in Aqueous Solutions and Colligative ... · PDF file3. Will a precipitate form if solutions of barium chloride ... aqueous solutions conduct electricity well;

Chapter 13 – Ions in Aqueous Solutions and Colligative Properties

This chapter is part of the foundational knowledge to understand how acids work (Chapter 14).

This chapter is relatively short (Test on Tuesday, March 28)

Colligative Property: a property that is determined by the number of particles present in a system, but that is independent of the properties of the particles themselves.

Dissociation

• Dissociation is separation of ions that occurs when an ionic compound dissolves.

+⎯⎯⎯→s aq + aq2H O –NaCl( ) Na ( ) Cl ( )

+⎯⎯⎯→s aq + aq2H O 2 –2CaCl ( ) Ca ( ) 2Cl ( )

Chapter 13Section 1 Compounds in Aqueous Solution

1 mol 1 mol 1 mol

1 mol 1 mol 2 mol

How can we determine if dissociation will occur?

What trend do you notice in the following information?

Compound/Molecule Dissociation Occurs

NaCl YesC6H12O6 No

KF YesCH4 NoNO2 No

Chapter 13

Dissociation of NaCl


Dissociation, continuedSample Problem AWrite the equation for the dissociation of aluminum sulfate, Al2(SO4)3 , in water. How many moles of aluminum ions and sulfate ions are produced by dissolving 1 mol of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mol of aluminum sulfate?


Dissociation, continuedSample Problem A SolutionGiven: amount of solute = 1 mol Al2(SO4)3

solvent identity = waterUnknown: a. moles of aluminum ions and sulfate ions

b. total number of moles of solute ions producedSolution:

s aq + aq2H O 3 2–2 4 3 4Al (SO ) ( ) 2Al ( ) 3SO ( )+⎯⎯⎯→

+ 3 2–2 4 3 41mol Al (SO ) 2 mol Al 3 mol. SOa +→

+ 3 2–42 mol Al 3 mol SO 5 mol of solute i. sb on+ =


Dissociation (dissolution)

1. Write the equation for the dissociation of the following in water, and then determine the number of moles of each ion produced as well as the total number of moles of ions produced.,

a. 1 mol of ammonium chlorideb. 1 mol of sodium sulfidec. 0.5 mol barium nitrate

Dissociation, continuedPrecipitation Reactions

• Although no ionic compound is completely insoluble, compounds of very low solubility can be considered insoluble for most practical purposes.


Soluble and Insoluble Ionic Compounds


Particle Model for the Formation of a Precipitate


Dissociation, continuedNet Ionic Equations• A net ionic equation includes only those

compounds and ions that undergo a chemical change in a reaction in an aqueous solution.

• Ions that do not take part in a chemical reaction and are found in solution both before and after the reaction are spectator ions.


Dissociation, continuedNet Ionic Equations, continued

Overall ionic equation

aq + aq + aq + aq s + aq + aq

2 3– 4 2–

3– 4

Cd ( ) 2NO ( ) 2NH ( ) S ( )CdS( ) 2NO ( ) 2NH ( )

+ +

+

→

aq + aq s 2 2–Cd ( ) S ( ) CdS( )+ →


net ionic equation

Writing a Net Ionic Equation



Sample Problem BIdentify the precipitate that forms when aqueous solutions of lead (II) nitrate and sodium chloride are combined. Write the equation for the possible double-displacement reaction. Then write the formula equation, overall ionic equation, and net ionic equation for the reaction.


Sample Problem B SolutionGiven: identity of reactants: lead (II) nitrate and sodium chloride

reaction medium: aqueous solutionUnknown: a. equation for the possible double-displacement

reaction b. identity of the precipitate c. formula equation d. overall ionic equation e. net ionic equation



Sample Problem B Solution, continuedSolution:a. equation for the possible double-displacement reaction


b. Table 1 reveals that zinc sulfide is not a soluble sulfide and is therefore a precipitate. Ammonium nitrate is soluble according to the table.

c. The formula equation


Pb(NO3)2(aq) + 2 NaCl(aq) à PbCl2(?) + 2 NaNO3(?)

Pb(NO3)2(aq) + 2 NaCl(aq) à PbCl2(s) + 2 NaNO3(aq)

Sample Problem B Solution, continuedd. The overall ionic equation


e. The sodium and chloride ions appear on both sides of the equation as spectator ions.

The net ionic equation:


Pb2+(aq) + 2 NO3

-(aq) + 2 Na+

(aq) + 2Cl-(aq) àPbCl2(s) + 2 Na+

(aq) + 2 NO3-(aq)

Pb2+(aq) + 2Cl-(aq) à PbCl2(s)

Practice Problems

Page 416 - #1-41. Will a precipitate form if solutions of potassium sulfate

and barium nitrate are combined? If so, write the net ionic equation for the reaction.

2. Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined? If so, write the net ionic equation

3. Will a precipitate form if solutions of barium chloride and sodium sulfate are combined? If so, identify the spectator ions and write the net ionic equation.

4. Write the net ionic equation for the precipitation of nickel (II) sulfide

• Ions are formed from solute molecules by the action of the solvent in a process called ionization.

• When a molecular compound dissolves and ionizes in a polar solvent, ions are formed where none existed in the undissolved compound.

• Hydrogen chloride, HCl, is a molecular compound that ionizes in aqueous solution.

• HCl contains a highly polar bond.

aq + aq2H O –HCl H ( ) Cl ( )+⎯⎯⎯→


Ionization

Ionization, continuedThe Hydronium Ion

• Some molecular compounds ionize in an aqueous solution to release H+.

• The H+ ion attracts other molecules or ions so strongly that it does not normally exist alone.

aq + aq2H O –3HCl H O ( ) Cl ( )+⎯⎯⎯→


• The H3O+ ion is known as the hydronium ion.

Strong and Weak Electrolytes

• Electrolytes are substances that yield ions and conduct an electric current in solution.

• The strength with which substances conduct an electric current is related to their ability to form ions in solution.

• Strong and weak electrolytes differ in the degree of ionization or dissociation.


Models for Strong and Weak Electrolytes and Nonelectrolytes


Strong and Weak Electrolytes, continuedStrong Electrolytes

• A strong electrolyte is any compound whose dilute aqueous solutions conduct electricity well; this is due to the presence of all or almost all of the dissolved compound in the form of ions.

• To whatever extent they dissolve in water, they yield only ions.

• HCl, HBr, HI• All soluble ionic compounds•Example:


aq + aq2H O –3HCl H O ( ) Cl ( )+⎯⎯⎯→

[HF] >> [H+] and [F–]

Strong and Weak Electrolytes, continuedWeak Electrolytes• A weak electrolyte is any compound whose dilute

aqueous solutions conduct electricity poorly; this is due to the presence of a small amount of the dissolved compound in the form of ions (only ~10% ionization.

• Some molecular compounds form aqueous solutions that contain not only dissolved ions but also some dissolved molecules that are not ionized.

+⎯⎯→←⎯⎯ aq + aq–3HF H O ( ) F ( )


Practice Problems

1. The following solutions are combined in a beaker: NaCl, Na3PO4, and Ba(NO3)2. a) Will a precipitate form? If so, provide the name and the formula of the precipitate. List all spectator ions in the system.

2. Acetic acid, CH3CO2H,is a week electrolyte. Write an equation to represent its ionization in water. Include the hydronium ion, H3O+

(aq)

3. Hydroiodic acid, HI(aq) is a strong acid and creates a strong electrolytic solution when mixed with water. Write an equation to represent its ionization in water. Include the hydronium ion, H3O+

(aq)

End of Chapter 13-1 Show

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Chapter 13 – Ions in Aqueous Solutions and Colligative ... · PDF file3. Will a precipitate form if solutions of barium chloride ... aqueous solutions conduct electricity well;